Wound dressing

ABSTRACT

A method and apparatus are disclosed for dressing a wound. The apparatus comprises an absorbent layer for absorbing wound exudate, a liquid impermeable, gas permeable filter layer over the absorbent layer, a cover layer comprising at least one orifice and a first liquid and gas permeable transmission layer underlying the absorbent layer. The transmission layer is in fluid communication with the filter layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/744,277, filed Sep. 20, 2010, which is the U.S. National Phase of PCTInternational Application No. PCT/GB2008/051090 filed on Nov. 20, 2008,designating the United States and published on May 28, 2009 as WO2009/066106, which claims priority to Great Britain Patent ApplicationNo. 0722820.8, filed Nov. 21, 2007, and Great Britain Patent ApplicationNo. 0817020.1, filed Sep. 17, 2008. The disclosure of these priorapplications are incorporated by reference in their entireties andshould be considered a part of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method and apparatus for dressing awound and a method for manufacturing a wound dressing. In particular,but not exclusively, the present invention relates to a wound dressinguseable during topical negative pressure (TNP) therapy in which thewound dressing itself acts as a waste canister to collect and storewound exudate removed from a wound site.

There is much prior art available relating to the provision of apparatusarid methods of use thereof for the application of topical negativepressure (TNP) therapy to wounds together with other therapeuticprocesses intended to enhance the effects of the TNP therapy. Examplesof such prior art include those listed and briefly described below.

TNP therapy assists in the closure and healing of wounds by reducingtissue oedema; encouraging blood flow and granulation of tissue;removing excess exudates and may reduce bacterial load and thus,infection to the wound. Furthermore, TNP therapy permits less outsidedisturbance of the wound and promotes more rapid healing.

In International patent application, WO 2004/037334, apparatus, a wounddressing and a method for aspirating, irrigating and cleansing woundsare described. In very general terms, the application describes thetreatment of a wound by the application of TNP therapy for aspiratingthe wound together with the further provision of additional fluid forirrigating and/or cleansing the wound, which fluid, comprising bothwound exudates and irrigation fluid, is then drawn off by the aspirationmeans and circulated through means for separating the beneficialmaterials therein from deleterious materials. The materials which arebeneficial to wound healing are recirculated through the wound dressingand those materials deleterious to wound healing are discarded to awaste collection bag or vessel.

In International patent application, WO 2005/04670, apparatus, a wounddressing and a method for cleansing a wound using aspiration, irrigationand cleansing wounds are described. Again, in very general terms, theinvention described in this document utilizes similar apparatus to thatin WO 2004/037334 with regard to the aspiration, irrigation andcleansing of the wound, however, it further includes the importantadditional step of providing heating means to control the temperature ofthat beneficial material being returned to the wound site/dressing sothat it is at an optimum temperature, for example, to have the mostefficacious therapeutic effect on the wound.

In International patent application, WO 2005/105180, apparatus and amethod for the aspiration, irrigation and/or cleansing of wounds aredescribed. Again, in very general terms, this document describes similarapparatus to the two previously mentioned documents hereinabove but withthe additional step of providing means for the supply and application ofphysiologically active agents to the wound site/dressing to promotewound healing.

The content of the above-noted references is included herein byreference.

However, the above described apparatus and methods are generally onlyapplicable to a patient when hospitalised as the apparatus used iscomplex, needing people having specialist knowledge in how to operateand maintain the apparatus, and also relatively heavy and bulky, notbeing adapted for easy mobility outside of a hospital environment by apatient, for example.

Some patients having relatively less severe wounds which do not requirecontinuous hospitalisation, for example, but whom nevertheless wouldbenefit from the prolonged application of TNP therapy, could be treatedat home or at work subject to the availability of an easily portable andmaintainable TNP therapy apparatus. To this end GB-A-2 307 180 describesa portable TNP therapy unit which may be carried by a patient andclipped to belt or harness. A negative pressure can thus be applied at awound site.

During TNP therapy a portable or non-portable therapy unit generates anegative pressure at a wound site. As fluid, including air as well aswound exudate material is removed from the wound site this must becollected in some manner remote from the wound site. With prior knowntherapy units ‘the collection and storage of wound exudate material istypically carried out by a waste canister connected to a pump unit ofthe therapy unit. The use of a canister, however, can result in thetherapy unit apparatus itself being quite bulky and expensive tomanufacture. Also replacing a canister or a bag in a canister in whichwound exudate is collected can be a time consuming and relativelyunhygienic process.

Prior known therapy units also tend to include a pump which is used togenerate the negative pressure. Such pumps can be costly to manufactureand are relatively heavy.

WO 2007/030601, which is incorporated herein by reference discloses aself-contained wound dressing with a micro pump. The pump for drawingwound fluid into a vacuum zone is included in a wound dressing itself.Nevertheless wound exudate from the dressing can only be removed via acomplex series of steps. The exudate removal process is also prone tocontamination since once an absorbent layer is fully saturated withwound exudate an access door must be opened in the wound dressing sothat the absorbent layer and micro pump can be removed. It will beappreciated that such exudate removal and pump removal can be timeconsuming and can lead to cross contamination between users. A furtherproblem is that the wound dressing is prone to over expansion andrupture.

SUMMARY OF SOME EXEMPLIFYING EMBODIMENTS

It is an aim of the present invention to at least partly mitigate theabove-mentioned problems.

It is an aim of certain embodiments of the present invention to providea method for providing negative pressure at a wound site to aid in woundclosure and healing in which wound exudate drawn from a wound siteduring the therapy is collected and stored in a wound dressing.

It is an aim of certain embodiments of the present invention to providea wound dressing which is able to be placed over a wound site and whichincludes an integrated pump to generate negative pressure at that woundsite. Also for certain embodiments the wound dressing can collect anywound exudate.

According to a first aspect of the present invention there is providedapparatus for dressing a wound, comprising:

-   -   an absorbent layer for absorbing wound exudate;    -   a liquid impermeable, gas permeable filter layer over the        absorbent layer; and    -   a cover layer comprising at least one orifice;    -   wherein the absorbent layer is in fluid communication with the        filter layer.

According to a second aspect of the present invention there is provideda method of applying topical negative pressure (TNP) at a wound site,comprising the steps of:

-   -   pumping wound exudate and air from a wound site, a peripheral        region around the wound site being sealed with a wound dressing;    -   collecting wound exudate, pumped from the wound site in an        absorbent layer of the wound dressing; and    -   exhausting gas through at least one orifice in a cover layer of        the wound dressing    -   and a filter layer in fluid communication with the absorbent        layer.

Certain embodiments of the present invention provide the advantage thata disposable wound dressing can be fixed over a wound site and cansimultaneously be used to provide negative pressure at the wound siteand collect and store wound exudate.

Certain embodiments of the present invention provide the advantage thata separate therapy unit is not required to generate negative pressure ata wound site and collect and store any wound exudate. Rather a wounddressing can carry out both a pumping and wound exudate collectingprocess. The wound dressing may then be a one use item which can bedisposed of subsequent to use. This reduces a risk of contamination.

Certain embodiments of the present invention provide the advantage thata wound dressing can be used to collect wound exudate generated during anegative pressure therapy process. A pump remote from the wound dressingcan be connected to the wound dressing and reused whilst the wounddressing itself is used to collect wound exudate and may then bedisposed of after use.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described hereinafter,by way of example only, with reference to the accompanying drawings inwhich:

FIG. 1 illustrates a wound dressing;

FIG. 2 illustrates a top view of a wound dressing;

FIG. 3 illustrates a portion of the wound dressing;

FIG. 4 illustrates an exploded view of a wound dressing with a mountedpump; and

FIG. 5 illustrates a view of a horizontal section through a wounddressing.

In the drawings like reference numerals refer to like parts.

DETAILED DESCRIPTION OF SOME EXEMPLIFYING EMBODIMENTS

FIG. 1 illustrates a cross section through a wound dressing 100according to an embodiment of the present invention. A plan view fromabove of the wound dressing 100 is illustrated in FIG. 2 with the lineA-A indicating the location of the cross section shown in FIG. 1. Itwill be understood that FIG. 1 illustrates a generalised schematic viewof an apparatus 100. It will be understood that embodiments of thepresent invention are generally applicable to use in topical negativepressure (TNP) systems. Briefly, negative pressure wound therapy assistsin the closure and healing of many forms of “hard to heal” wounds byreducing tissue oedema; encouraging blood flow and granular tissueformation; removing excess exudate and reducing bacterial load (and thusinfection risk). In addition, the therapy allows for less disturbance ofa wound leading to more rapid healing.

The wound dressing 100 can be located over a wound site to be treated.The dressing 100 forms a sealed cavity over the wound site. Optionallywound packer can be used within a wound cavity below the dressing. Aptlythe packer material can be a gauze or reticulated PU foam material.

It is envisaged that the negative pressure range for the apparatusembodying the present invention may be between about −50 mmHg and −200mmHg (note that these pressures are relative to normal ambientatmospheric pressure thus, −200 mmHg would be about 560 mmHg inpractical terms). Aptly the pressure range may be between about −75 mmHgand −150 mmHg. Alternatively a pressure range of up to −75 mmHg, up to−80 mmHg or over −80 mmHg can be used. Also aptly a pressure range ofbelow −75 mmHg could be used. Alternatively a pressure range of over−100 mmHg could be used or over −150 mmHg.

As illustrated in FIG. 1 a lower surface 101 of the wound dressing 100is provided by an optional wound contact layer 102. The wound contactlayer 102 can be a polyurethane layer or polyethylene layer or otherflexible layer which is perforated, for example via a hot pin process orin some other way, or otherwise made permeable to liquid and gas. Thewound contact layer has a lower surface 101 and an upper surface 103.The perforations 104 are through holes in the wound contact layer whichenables fluid to flow through the layer. The wound contact layer helpsprevent tissue ingrowth into the other material of the wound dressing.The perforations are small enough to meet this requirement but stillallow fluid through. The wound contact layer also helps hold the wholewound dressing together and acts as a carrier for an optional lower andupper adhesive layer (not shown). For example, a lower pressuresensitive adhesive may be provided on the underside surface 101 of thewound dressing whilst an upper pressure sensitive adhesive layer may beprovided on the upper surface 103 of the wound contact layer. Thepressure sensitive adhesive which may be a silicone or acrylic basedadhesive or other such adhesives may be formed on both sides oroptionally on a selected one or none of the sides of the wound contactlayer. When a lower pressure sensitive adhesive layer is utilised thishelps adhere the wound dressing to the skin around a wound site.

An optional layer 105 of porous material such as a foam layer or thelike is located above the wound contact layer. This porous layer allowstransmission of fluid including liquid and gas away from a wound siteinto upper layers of the wound dressing, The layer 105 also helpsdistribute pressure generated by a pump, mentioned in more detail below,so that a whole wound site sees an equalised negative pressure.Reticulated foam or a non-woven material which might be natural orsynthetic can be used as the porous material of the porous layer 105.

A layer 110 of absorbent material is provided above the transmissionlayer 105 or where no lower transmission layer is used on the woundcontact layer or where no transmission layer 105 or wound contact layer102 are used the absorbent layer lower surface forms the wound contactlayer. The absorbent material which may be a foam or non-woven naturalor synthetic material and which may optionally include or besuper-absorbent material forms a reservoir for fluid, particularlyliquid, removed from the wound site. The material of the absorbent layeralso prevents liquid collected in the wound dressing from flowing in asloshing manner. The absorbent layer 130 also helps distribute fluidthroughout the layer via a wicking action so that fluid is drawn fromthe wound site and stored throughout the absorbent layer. This preventsagglomeration in areas of the absorbent layer. Since in use theabsorbent layer experiences negative pressures the material of theabsorbent layer is chosen to absorb liquid under such circumstances.Superabsorber material is an example of such a material. Nonsuperabsorber material can be utilised however even where significantnegative pressures are envisaged. The material of the absorbent layerdoes not need to be hydrophilic. Aptly a material with connective openvoids can be used. Aptly a material is used that can resist thecompressive force of the negative pressure e.g. precompressed FT11M foammanufactured by Foam Techniques. Aptly the absorbent material isselected so that fluid is prevented from draining back out when thedressing is removed. It is to be noted that if a superabsorber is usedsuch a material is able to expand against the compressive force of thenegative pressure.

A further optional layer 112 of porous material such as a foam layer orthe like is located above the absorbent layer 110. This porous layerallows transmission of fluid including liquid and gas away from a woundsite into upper layers of the wound dressing. The layer 112 also helpsdistribute pressure generated by a pump, mentioned in more detail below,so that a whole wound site sees an equalised negative pressure.Reticulated foam or a non-woven material which might be natural orsynthetic can be used as a porous material of the porous layer 112. Thematerial may the same or different from the previously mentioned layer105 of porous material.

A filter layer 130 is provided over the absorbent layer 110. The filterlayer permits moisture vapour and gas but no liquid through. A suitablematerial for the filter material of the filter layer 130 is 0.2 micronGore™ expanded PTFE from the MMT range. Larger pore sizes can also beused but these may require a secondary filter layer to ensure fullbioburden containment. As wound fluid contains lipids it is preferable,though not essential, to use an oleophobic filter membrane for example1.0 micron MMT-332 prior to 0.2 micron MMT-323. This prevents the lipidsfrom blocking the hydrophobic filter.

It will be understood that other types of material could be used for thefilter layer. More generally a microporous membrane can be used which isa thin, flat sheet of polymeric material, this contains billions ofmicroscopic pores. Depending upon the membrane chosen these pores canrange in size from 0.01 to more than 10 micrometers.

Microporous membranes are available in both hydrophilic (waterfiltering) and hydrophobic (water repellent) forms. Aptly the wounddressing 100 according to certain embodiments of the present inventionuses microporous hydrophobic membranes (MHMs). Numerous polymers may beemployed to form MHMs. For example, PTFE, polypropylene, PVDF andacrylic copolymer. All of these optional polymers can be treated inorder to obtain specific surface characteristics that can be bothhydrophobic and oleo phobic. As such these will repel liquids with lowsurface tensions such as multi-vitamin infusions, lipids, surfactants,oils and organic solvents.

MHMs block liquids whilst allowing air to flow through the membranes.They are also highly efficient air filters eliminating potentiallyinfectious aerosols and particles. A single piece of MHM is well knownas an option to replace mechanical valves or vents. Incorporation ofMHMs can thus reduce product assembly costs improving profits andcosts/benefit ratio to a patient.

The filter layer 130 thus enables gas to be exhausted upwards throughthe wound dressing. Liquid, particulates and pathogens however arecontained in the dressing.

A gas impermeable sealing layer 140 extends across the width of thewound dressing. The sealing layer which may, for example, be apolyurethane film having a pressure sensitive adhesive on both sides isimpermeable to gas and this layer thus operates to seal a wound cavityover which the wound dressing is placed. In this way an effectivechamber is made beneath the sealing layer and between the sealing layerand a wound site where a negative pressure can be established. Thesealing layer 140 is sealed to the filter layer 130. For example viaadhesive or welding techniques. Gas leaving the dressing thus passesthrough the filter layer and sealing layer.

Aptly the material of the sealing layer can have a high moisture vapourpermeability for example Elastollan (Trade name) SP9109 manufactured byBASF. A dotted pattern spread acrylic adhesive can optionally be used tohelp improve moisture vapour permeability. An advantage of using a highmoisture vapour permeability material as the sealing layer 160 is thatthe fluid handling capacity of the dressing may be increasedsignificantly by the action of moisture transpiring through the film anddispersing into the atmosphere. Advantageously, transpiration rates canbe easily achieved of the order of 3000 grams/centimeter square/24 hoursas a result of the high humidity achieved in the dressing and intimatecontact of material achieved during use of the apparatus at a negativepressure of up to 250 mmHg below atmospheric pressure.

As illustrated in FIG. 1 a grid array of through holes 141 are providedin the sealing layer. These enable fluid including gas and liquid topass through the sealing layer 140. Alternatively where a separate coverlayer and sealing layer are used the sealing layer may extend aroundonly a circumferential area of the underlying layers where it sealsbetween an outer layer (mentioned below in more detail) and the filterlayer. As a result any gas leaving the wound site must leave via thefilter layer. Liquid is retained in layers below the filter layer.

A layer 150 of porous material such as a foam layer or the like islocated above the sealing layer 140. This porous layer allowstransmission of fluid including liquid and gas away from a wound site.The layer 150 also helps distribute pressure generated by a pump,mentioned in more detail below, so that a whole wound site sees anequalised negative pressure. Reticulated foam or a non-woven materialwhich might be natural or synthetic can be used as the porous materialof the layer 150. The material may be the same or different from thematerial of the underlying layers 105, 112.

A cover layer 160 covers the absorbent layer of the wound dressing 100.The cover layer which, for example, may be a polyurethane film acts as abacterial barrier and helps hold in liquid to stop fouling. The coverlayer also provides integrity for the dressing and is impermeable tomoisture vapour and gas. The cover layer helps hold the wound dressingtogether thus providing structural integrity. An upper surface 171 alsopresents a bacteria free non soiling surface. As an alternative thematerial of the cover layer can have a high moisture vapourpermeability, for example Elastollan (Trade name) SP9109 manufactured byBASF. A dotted pattern spread acrylic adhesive can optionally be used tohelp improve moisture vapour permeability. An advantage of using a highmoisture vapour permeability material as the cover layer 160 is that thefluid handling capacity of the dressing may be increased significantlyby the action of moisture transpiring through the film and dispersinginto the atmosphere. Advantageously, transpiration rates can be easilyachieved of the order of 3000 grams/centimeter square/24 hours as aresult of the high humidity achieved in the dressing and intimatecontact of material achieved during use of the apparatus at a negativepressure of up to 250 mmHg below atmospheric pressure.

A single aperture 165 farmed as a single hole or close arrangement ofholes is formed in a central region of the upper cover layer. Theaperture 165 is in fluid communication with an inlet to a pump 170 whichis mounted on the upper surface 171 of the cover layer. In operation thepump 170 pumps fluid through the wound dressing from a wound site belowthe wound contact layer 102 upwards through the first transmission layer105, absorbent layer 110, further transmission layer 112, filter layer130, sealing layer 140, and further transmission layer 150.

Turning to FIG. 2 which illustrates a wound dressing 100 in accordancewith an embodiment of the present invention one can see the uppersurface 171 of the cover layer 170 which extends radially outwardly awayfrom a centre of the dressing into a border region 200 surrounding acentral raised region 201 overlying the foam layers and layer 110 ofabsorber. FIG. 2 also helps illustrate the location of the pump 170 onthe cover layer. As indicated in FIG. 2 the general shape of the wounddressing is a square having equal side lengths with rounded cornerregions 202. It will be appreciated that wound dressings according toother embodiments of the present invention can be shaped differentlysuch as rectangular, circular or elliptical dressings.

FIG. 3 illustrates an expanded view of the border region 200 of thewound dressing 100 illustrated in FIGS. 1, 2 and 3. As seen, the coverlayer 160 extends over the foam transmission layer 150 into an edgeregion. Here the cover layer is secured to the sealing layer 140 and thewound contact layer 102. FIG. 3 also helps illustrate how theperforations 104 in the wound contact layer 102 extend around the foamlayer 105 and absorbent layer 110. It will be noted that a space 301 isindicated in FIG. 3 underneath the sealing layer 140 and above the woundcontact layer 102 and ends of the transmission layers 105, 112 andabsorbent layer 110. The space 301 is shown for illustrative reasonsonly and in practice the transmission layers and absorbent layers willbe bevelled somewhat so as to reduce the space. A further space 302 islikewise illustrated in FIG. 3 above the sealing layer and below theinner surface of the cover layer. Again this is included forillustration only and in practice these spaces will be avoided due to anipping process in the method of manufacture. It will also beappreciated by those skilled in the art that when put in use the wounddressing will be subject to a negative pressure within a region definedby the inner surface of the cover layer. Such a negative pressure willtend to collapse any remaining spaces.

It will be understood that according to embodiments of the presentinvention the wound contact layer is optional. This layer is, if used,porous to water and faces an underlying wound site. A lower porous layer105 such as a reticulated PU foam layer is used to distribute gas andfluid removal such that all areas of a wound are subjected to equalpressure. The sealing layer together with the filter layer forms asubstantially liquid tight seal over the wound. Thus as the pump 170pumps a negative pressure is generated below the sealing layer. Thisnegative pressure is thus experienced at the target wound site. Fluidincluding air and wound exudate is drawn through the wound contact layerand reticulated foam layer 105. The wound exudate drawn through thelower layers of the wound dressing is dissipated and absorbed into theabsorbent layer where it is collected and stored. Air and moisturevapour is drawn upwards through the wound dressing through theintermediate transmission layer 112 and through the filter layer andsealing layer. The filter layer and sealing layer are secured togetherso as to prevent upward movement through the wound dressing of anythingother than moisture vapour and air. This air and moisture vapour isdrawn upwards by the pump 170 into the fluid inlet 300. The pumpexhausts the fluid as air and moisture vapour through a fluid exit (notshown).

It is to be noted that the upper transmission layer 150 and cover layer160 are optional. Their use is helpful during multi orientation use whenportions of the filter layer might otherwise become occluded. When aseparate cover layer 160 is not utilised the sealing layer also performsthe extra function of covering the dressing and will thus additionallyact as a cover layer.

It will be appreciated by those skilled in the art that rather than havea cover layer overlying the filter layer the cover layer may itself beoverlain by a filter layer. The cover layer may thus be the outermostlayer of the wound dressing or the filter layer may be the outermostlayer of the wound dressing. Further outer layers (not shown) mayoptionally be used so long as they are gas and water vapour permeable.

As still further options the dressing can contain anti-microbial e.g.nanocrystalline silver agents on the wound contact layer and/or silversulphur diazine in the absorbent layer. These may be used separately ortogether. These respectively kill micro-organisms in the wound andmicro-organisms in the absorption matrix. As a still further optionother active components, for example, pain suppressants, such asibuprofen, may be included. Also agents which enhance cell activity,such as growth factors or that inhibit enzymes, such as matrixmetalloproteinase inhibitors, such as tissue inhibitors ofmetalloproteinase (TIMPS) or zinc chelators could be utilised. As astill further option odour trapping elements such as activated carbon,cyclodextrine, zealite or the like may be included in the absorbentlayer or as a still further layer above the filter layer.

FIG. 4 illustrates an exploded view of the wound dressing illustrated inFIGS. 1, 2 and 3. As illustrated in FIG. 4 the lower-most layer of thewound dressing is a perforated wound contact layer 102. It will beappreciated that prior to use a still lower protective layer may besecured to the lower surface 101 of the wound contact layer. Theprotective paper (not shown) is removed immediately prior to applicationof the wound dressing over a wound site. During manufacture a centralregion 400 of the wound contact layer 102 is made slightly concave so asto provide a dished upper surface 103 for the wound contact layer.

A transmission layer 105 is duly located in the dished central region400 of the wound contact layer. The foam layer includes a substantiallyrectangular base region 401 together with an array of upstanding columns402. As illustrated in FIG. 4 an array of 8×8 columns 402 may be used.It will be appreciated that other numbers of columns may be utilised.The columns 402 have a substantially circular cross section although itwill be appreciated that column elements having different cross sectionshapes could be used. The column elements 402 and base section 402 areaptly integrally formed although these could be separately formed withthe column elements being secured to the base section in someappropriate way such as via adhesive techniques.

The absorbent layer 110 is located above the transmission layer 105. Theabsorbent layer 110 is a layer of absorbent material and includesthrough holes 403 formed in a substantially rectangular block 404 ofabsorbent material. The through holes are set out in an 8×8 array tocoincide with the upstanding columns 402 in the underlying transmissionlayer. It will be appreciated that the number and pattern of throughbores 403 is selected to tally with the shape and number and arrangementof the columns.

The intermediate transmission layer 112 is a substantially rectangularbase section 405 of porous material such as reticulated foam with anarray of columns 406 extending downwardly from a lower surface of thebase 405. The columns 406 coincide with locations of the through bores403 in the absorbent layer. It will be appreciated that the columns 406of the intermediate transmission layer 112 may be integrally formed withthe base portion 405 of the transmission layer or may be secured in somefashion thereto. The height of the columns 402,406 of the lower andintermediate transmission layers respectively is such that an uppercontact surface of the columns 402 of the lower transmission layer and alower contact surface of the columns 406 of the intermediatetransmission layer contact when the wound dressing is put together.These thus provide fluid transmission paths through the absorber layerso that fluid, including air and wound exudate and liquid, is drawn fromthe lower region upwardly through the absorbent layer when the pump 170is operating.

A rectangular layer of filter material 130 is located above the uppersurface of the base section 405 of the intermediate transmission layer.The filter layer blocks movement therethrough of liquid. The filterlayer is aptly a 0.2 micron Gore™ expanded PTFE sheet.

A sealing layer 140 is located over the filter layer 130. The sealinglayer has a border region and a generally concave central region 407.The underside of the sealing layer 140 is thus recessed. An array ofapertures set out in a 5×5 grid array is made through the sealing layer140. The sealing layer away from the apertures is gas and fluid tight.If a material having a high moisture vapour permeability is optionallyused then the sealing layer will of course be permeable to moisturevapour. Fluid, including liquid and gas, can of course penetrate throughthe perforations. The filter layer 130 which is secured on the undersideof the sealing layer, however, prevents liquid penetration through theapertures and to an extent prevents penetration of air through theapertures. Moisture vapour can penetrate through the apertures.

An upper transmission layer formed as a sheet of reticulated foam islocated over the central region of the upper surface of the sealinglayer 140. The upper transmission layer acts as a manifold and diffuserto help spread the negative pressure generated by the pump 170.

A cover layer 160 is located over the sealing layer and uppertransmission layer 150. The cover layer has a border region 200 and acentral raised region 201. The underside of the cover layer thuspresents a central dished region to receive the upper transmissionlayer, raised central region of the sealing layer and the filter layer,intermediate transmission layer, absorbent layer and lower transmissionlayer. A central aperture 165 is made in the centre of the upper surfaceof the cover layer. The central aperture 165 is located to coincide witha fluid inlet 300 of the pump 170. Thus in use when a pump 170 is in usea negative pressure is generated under the cover 160. This negativepressure is distributed throughout the wound dressing and at a targetwound site located under the wound contact layer. As the negativepressure is established and maintained wound exudate and air is drawnupwards away from the wound site through the wound dressing. Liquid andair is drawn upwards through the wound contact layer into the base ofthe lower transmission layer 105 and upwards through the connectingcolumns in the lower transmission layer and intermediate transmissionlayer. It will be appreciated of course that columns having a heightsufficient to bridge the whole of the absorbent layer could be providedon either the upper surface of the lower transmission layer 105 or thelower surface of the intermediate transmission layer 112. Alternativelythe apertures 403 in the absorbent layer may be filled with transmissivematerial such as foam cylinders when the wound dressing is manufactured.Any wound exudate being drawn upward through the wound dressing isdissipated outwardly from the absorbent material in the aperture regionsof the absorbent layer. The liquid is thus collected and stored in theabsorbent layer. Air and moisture vapour carries on upwards through thefilter layer 130 and sealing layer 140 and is evacuated by the pump 170.

FIG. 5 illustrates a horizontal cross section through the wound dressingillustrating an upper surface 404 of the absorbent layer includingapertures 403. Each of the apertures 403 is filled with absorbentmaterial such as columns 402 from the lower intermediate layer.

It will be appreciated that according to certain embodiments of thepresent invention fluid communication paths through which fluid can betransmitted from the lower transmission layer to the intermediatetransmission layer can be made by pinching together peripheral regionsof the lower and intermediate regions. Fluid transmission would thusproceed around the peripheral edges of the wound dressing. Such fluidpaths may replace the fluid paths formed by the columns passing throughapertures in the absorbent layer or may alternatively take the place ofsuch passageways. This would maximise the quantity of absorber materialin the layer 110 in the resultant wound dressing.

It is to be noted that according to certain other embodiments of thepresent invention a remote pump may be mounted to a border region of thewound dressing rather than onto the top surface. In such case tubes maybe connected directly to the pump. Subsequent to a single use the wounddressing and pump may thus be discarded. As an option the tubes may beprovided with a click fit connector or other easy fit connector whichcan be connected to corresponding mating connectors joined viacorresponding tubes to a remote pump. In this way a remote pump may bereused whilst the wound dressing itself including connecting tubes andconnectors is disposable after a single use.

It will be appreciated that alternatively the tubes could be provided bya single dual lumen tube. As a still further alternative the tubes maybe provided by a single continuous looped tube, the tube then passingthrough pinch rollers for a peristaltic pump.

It will be understood that for embodiments of the present inventionwhich include a pump mounted on the cover layer or on a peripheralborder area of the dressing an integral power source and controlcircuitry can be included. Alternatively the power source can beexternal to the pump and remotely mounted. A remote power source and/orcontrol circuitry improves the disposability of the dressing and permitsbattery recharge if spare batteries are used.

It is to be noted that in use the dressing may be used “up-side down”,at an angle or vertical. References to upper and lower are thus used forexplanation purposes only.

Where a separate cover layer and sealing layer are utilised such layersmay be manufactured from the same or different materials.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

What is claimed is:
 1. A system for applying negative pressure to awound site, the system comprising: a wound dressing comprising: aperforated wound contact layer for contacting the wound site; a firsttransmission layer for distributing negative pressure overlying thewound contact layer; an absorbent layer for absorbing wound exudate viathe wound contact layer and the first transmission layer, the absorbentlayer overlying the first transmission layer; a second transmissionlayer overlying and in direct contact with the absorbent layer, thesecond transmission layer adapted to distribute negative pressure; acover layer overlying the second transmission layer, the cover layercomprising at least one aperture and further comprising a border regionand a central region, wherein the cover layer is directly secured to thewound contact layer in the border region; a filter provided over thesecond transmission layer and positioned at the at least one aperture inthe cover layer, and; a negative pressure source configured to be influid communication with the at least one aperture in the cover layervia a conduit, the negative pressure source configured to generatenegative pressure under the cover layer.
 2. The system of claim 1,wherein a peripheral region of the first transmission layer and aperipheral region of the second transmission layer are pinched togetherto provide a fluid communication path around a peripheral edge of theabsorbent layer.
 3. The system of claim 2, wherein the negative pressuresource is configured to communicate negative pressure to the wound sitevia at least one fluid transmission path passing through theperforations in the wound contact layer and through the peripheralregion of the first transmission layer and the peripheral region of thesecond transmission layer alongside at least one edge of the absorbentlayer.
 4. The system of claim 1, wherein the filter is in fluidcommunication with the absorbent layer.
 5. The system of claim 1,wherein the wound dressing comprises a square shape having equal sidelengths with rounded corner regions.
 6. The system of claim 1, whereinthe at least one aperture of the cover layer is positioned in thecentral region of the upper surface of the cover layer.
 7. The system ofclaim 1, further comprising an adhesive provided on an underside of thewound dressing.
 8. The system of claim 7, further comprising areleasable protective layer on a lower surface of the wound contactlayer.
 9. The system of claim 1, wherein the central region of the coverlayer is a central raised region, and wherein the second transmissionlayer acts as a manifold and a diffuser to help spread the negativepressure generated by the pump.
 10. The system of claim 1, wherein thefilter comprises a hydrophobic material.
 11. The system of claim 1,wherein the filter comprises PTFE.
 12. The system of claim 1, the filtercomprising a first filter layer and a secondary filter layer.
 13. Thesystem of claim 1, wherein the filter is secured to the cover layer withan adhesive.
 14. The system of claim 1, wherein the wound dressing isdisposable after a single use.
 15. The system of claim 14, wherein thenegative pressure source is reusable.
 16. The system of claim 1, whereinthe wound contact layer and the cover layer hold the wound dressingtogether.
 17. The system of claim 1, wherein the filter is providedunder the cover layer.
 18. The system of claim 1, wherein the filter isprovided over the cover layer.